You can do the Shadowbringers MSQ in few days if you watch cut scenes, or over 2 weeks if you do sidequests. The patches have a lot of lengthy cut scenes but should also not take that long.

The raids like alex, omega and the ones from shadowbringers should only take a day each.

You can skip the optional dungeons, trials and alliance raids and do them later without much problem. Most optional dungeons involve you revisiting a dungeon you already did with some differences and trials don't need to happen right away to fit with the lore.

Alliances raids hardly relate to the main story so that even if you do it years later, they would probably still make sense. The exceptions are Crystal Tower (which you will understand why now) and the ones from Stormblood because a few references won't make sense once the events of Endwalker start.